The present invention relates generally to cellular, foamed or air-entrained concrete, or concrete containing air cells or voids throughout its volume, and particularly to improved foam and foam/cement mixtures for making cellular concrete.
There have been ongoing efforts over the last fifty years to produce stronger, more durable concrete with improved properties. Water is normally mixed with cement and aggregates to produce a flowable concrete mixture which can be properly extended and shaped prior to setting. However, the water to cement ratio is critical in determining the strength of the hardened concrete, with a lower water content producing stronger concrete. Simply reducing the water content is not possible, since the concrete mixture will no longer be readily workable.
Various proposals have been made in the past to reduce the water to cement ratio in making concrete. The main technique currently used is the use of superplasticizers or chemical agents to reduce the water to cement (w/c) ratio. This chemical method can generally reduce the water requirements of a concrete mix by 10% to 30%. There are three basic types of superplasticizer. The first type are anionic materials which create negative charges on cement particles, causing them to repel each other, thereby reducing surface friction and making the mixture more workable at lower water concentrations. These materials have no effect on the hydration process. Because of the short workability time of such materials, they normally must be added at the job site.
The second type of superplasticizers are materials designed to coat the cement particles. These materials both improve plasticity of the cement paste, thus allowing a lower water content, and also control the hydration process. This enables such materials to be used at higher concrete temperatures, often reducing or eliminating the need for ice.
The third generation or type of superplasticizers are also designed to coat the cement particles, but also can maintain initial setting characteristics similar to normal concrete while producing a highly plastic mix at a very low water to cement ratio. These materials may be batch plant added and controlled.
Although water reducing superplasticizers do increase plasticity of the concrete mixture, as well as allowing the water to cement ratio to be reduced, they are subject to some disadvantages. Superplasticizers typically allow a reduction to about 0.40 in the water to cement ratio, and provide a corresponding increase in concrete strength, at 100 psi for each 0.01 decrease in water to cement (w/c) ratio. However, some superplasticizers tend to retard the set of the concrete. Additionally, superplasticizers in general produce increased segregation, particularly when sand gradation is poor, and undermine the quality of air entrapment. Superplasticizers also tend to cause more pronounced bleeding, and thus more capillaries, and produce concrete with decreased resistance to water absorption.
Cellular concrete also has the potential for reducing water content in cement mixes. However, up to now it has not been possible to achieve this objective to any extent, due to the problems in producing a sufficiently durable bubble in the cement mix. Thus, although it has been known for some time that bubbles introduced into a cement mix can form small air cells in the concrete, the fragile nature of the bubbles has necessitated introduction of the bubbles at the job site and also limited the volume of concrete that can be effectively replaced by air cells.
If the foam is injected into concrete in a concrete mixer, the bubbles will often have insufficient strength to withstand mixing for several hours during transportation to a construction site. If the bubbles break, water contained in the bubbles will mix in with the concrete, altering the concrete consistency and producing undesirable mix properties, such as high slump. Additionally, the known foam generating devices typically produce foams which contain too much water. Thus, the prior art foams, when mixed with concrete, produce bubbles which break too easily, either during mixing or working of the concrete as it is transported, or during placing and finishing of the concrete. Thus, previous attempts to produce lighter weight high strength concrete structures have been unsuccessful due to breaking and coalescing of the bubbles which have produced cellular concretes with less controllable air content and workability at desired slump.
Thus, up to now, it has not been possible to produce a foam which is sufficiently stable to withstand the effects of mixing of concrete for extended periods of several hours or more during transportation and subsequent agitation of the concrete as it is placed and finished, without collapse of a substantial portion of the bubbles mixed with the concrete. Further, it has not been possible to produce a cement and foam mixture with significantly lower w/c ratio and high strength.